Most electron imaging detectors used in electron microscopy employ some kind of energy converter which is thin. For instance, film, which converts energy from the incoming electrons of the electron microscope image into excited states in silver halide molecules, allows the electrons to by-and-large pass through the detection medium and continue on into vacuum behind the detection volume. With appropriate design of the mechanical structures behind the film, backscatter is minimized. In recent years, alternatives to film have been developed, most notably fiber-optically-coupled CCD cameras which employ a thin electron-transmissive single-crystal or powder phosphor scintillator to convert electron energy into light, which is then collected and transmitted to the CCD image sensor by the fused-fiber-optic plate. The problem with these detectors is that electrons which have traversed the scintillator have a finite probability, typically 25-35% of being scattered in such a manner as to return to the scintillator a second time or multiple times. The return traversal can happen at a considerable distance from the original entrance position, with the average distance growing as a function of incident electron energy. The combined effect of variability in number of returning electrons, the amount deposited by returning electrons and the uncertainty in position of returning electrons results in a loss of image information content, reducing signal-to-noise ratios (SNR) by a factor of 2-10. Given the extreme criticality of SNR for many low-dose electron microscopical applications (e.g. “single-particle” protein structure determination, cellular tomography and electron protein crystallography), there is a need for a way to either eliminate the backscatter, or to eliminate the backscatter signal from the total signal. Various methods of reducing actual backscatter have been tried. Transmission scintillators with low-backscatter mirrors as beam dumps, high numerical aperture lenses and high QE backside CCDs have been tried as a way to reduce backscatter but aren't completely successful and are very expensive due to the cost of the lens and the backside CCD (see U.S. Pat. No. 5,517,033. All references are incorporated herein by reference). Electron image deceleration has been tried as a way to prevent the traversal and thereby eliminate the possibility. This technology is promising but also expensive and problematic due to the complexity of floating a camera at high (˜250 kV) voltage. (See e.g., U.S. Pat. Nos. 6,414,309, 5,998,790). Elimination of the scintillator and fiber-optic has been proposed in conjunction with the use of semiconductor devices with better radiation tolerance than CCDs, namely active pixel sensors as have been developed for use in high-energy physics. In these devices backscatter is reduced somewhat by the lower atomic number of the silicon substrate (to roughly 10%). The large lateral travel of electrons in the light substrate, however, coupled with the higher scintillation efficiency of lower-energy returning electrons, combine to reduce SNR still by a significant factor. Partial reduction of substrate thickness and elimination of a backing behind the sensor can reproduce some of the benefits of a transmission detector but larger size devices may limit the extent of thinning which is possible which maintaining sufficient device robustness.
Backscatter discrimination would seek to eliminate the effect of backscatter even in situations in which backscatter cannot be physically eliminated or can only be reduced. Removal of the backscatter signal from the total image signal would remove the primary shortcoming of solid state detectors for use in low-dose high-resolution structural biology work.